I think there's two big factors.
One is book vs script length. A film is 100 pages. Novels are 250+. This means big story-arch rewrites. It can really kill plot & character development. This is where series open up a lot of possibility.
That's usually the biggest adaption, but there are lots of other big things that must change. Novels can do character thoughts, exposition and other things well. For example, a strategic battle scene will work in a book. In a film, it's near impossible so they focus on action and drama. Films do visual representation, so action-rich scenes work. They get characters for "free," actors will represent a highly nuanced person in a way you immediately understand.
These add up to a big re-write, that cannot be avoided.
Writers like Asimov are the very best storytellers. They work on their own schedule, write what comes to them, discard bad work... If you handed a good novelist the task of writing a LOTR prequel novel about Frodo's parents, by October... It probably won't be as good as Tolkien's.
You just can't replicate genius on commission.
This is really evident in Game of Thrones. The early seasons capture what's great about the books. A politically driven story that isn't boring, a novel approach to magical realism... Early seasons were loyal to the book. The latest season was mostly "off-script." The politics got stupid. Characters wandered closer to fantasy tropes. The "realism" part of magical realism was gone. Apart from big plot points, it wasn't written by GRRM and it showed. So, it is not good in the ways he's good. It was also written much quicker, on a schedule.
It'll be hard to capture Asimov in a series, harder than GRRM I expect. His stories are just not story driven enough.
His main plot device is not a spaceship or a time machine, it's a fictional theory, psychohistory. How do you capture that on film, line charts?
Good luck to them though. I love sci fi.